The present invention relates to a method of preparing high swell resins during the polymerization of ethylene to high density polyethylene with high efficiency Ziegler catalysts.
High swell resins are used primarily in blow molding applications, where the increase in parison diameter after leaving the die can be critical in respect to properly filling the mold and getting the desired bottle weight.
It has been difficult to prepare high swell resins by usual polymerization procedures. As pointed out by Schreiber (J. Appl. Pol. Sci 9 887 [1965]), just broadening molecular weight distribution (MWD) is not adequate to increase swell appreciably; a relatively small amount of very long chains must be introduced. If these are long enough to crystallize in two different domains, they form a lightly crosslinked, elastically deformable network which contributes to increased swell.
One approach has been to incorporate a small amount of high molecular weight polymer by melt blending. However, it is difficult to obtain the degree of dispersion which is necessary for high swell. In addition, melt blending requires high energy input and tends to degrade the polymer, particularly the high molecular weight fraction.
Berger, in U.S. Pat. No. 3,392,213 teaches to make bimodal polyethylene by polymerizing ethylene in a two-zone reactor wherein hydrogen is used to control molecular weight in the first zone and a higher molecular weight is formed in the second zone by reducing the amount of hydrogen therein. The separate zones are, for practical reasons, separate reactors.